This is a fun moment in time and on the whole it has been well caught. The exposure is good, given the bright background you were working against, and I love the shallow depth of field. It makes your son really stand out against what could otherwise be a distracting background.
If I was to be critical I would only have two concerns with this image. The first is the cropping; ideally I would have liked the camera to have been down slightly so that we can see the hand in front.
The second concern is the dark shadowing on that forward arm. Depending on whether you were working in JPEG or RAW, I would be using an adjustment to select that arm and lighten it about 10% while at the same time trying to remove some of the blue from the skin tone.
Other than those concerns, I think this is a great moment captured and I hope you take many more like it in years to come (assuming he hasn’t trashed your camera with a sand ball first!).